Air circulating systems for use in residential and commercial heating, ventilating, and air conditioning (HVAC) systems typically include a fan (the term "fan" including propeller fans, centrifugal blowers, and comparable air movers), ducting and/or duct fittings to connect the fan to the HVAC system, a filter holder configured for removal and replacement of an air filter, and an air filter.
Air filters such as those used in HVAC systems are provided with one or more types of filtering media or materials. Depending upon the type of substance sought to be filtered or removed from the air (e.g. dust or other debris particles, tobacco smoke particles, pollens, bacterial organisms, odoriferous gasses, etc.), and/or the useful filter life required, different filtering materials may be selected. Examples of such filtering materials are wet or dry laid paper which may include glass fibers, wet or dry laid fiberglass, synthetic microfibers in woven layers or nonwoven batts with or without tackifiers, open or closed cell polymeric foams, metal meshes, activated carbon, and biostats such as potassium permanganate.
A problem which sometimes arise when selecting a specific filtering material for a particular filtering requirement is that filtering performance for other coexisting requirements may be compromised. Accordingly, it would be desirable to combine different types of filtering materials within a filtering device which will then meet more than one filtering requirement including not just particulate removal but removal of odors and neutralization of bacterial organisms as well.
For structural integrity and for compactness (e.g., to fit within existing filter housings already installed in existing HVAC systems), it would also be desirable to provide a filtering device configured with more than one filtering material, or layer of filtering material, joined to each other and to a common support grid.
To help prevent distortion of filter elements in the presence of high pressure drops across them (caused, e.g., by high air flow rates and/or at least partial clogging of a filter element by filtrate), it would additionally be desirable for filter elements to be supported by adjacent filter or support screen elements as well as for the filtering device, as an assembly of filtering and support elements, to have a high bending modulus as provided by, for example, gluing or otherwise securing elements to each other whereby mechanical loads are shared by all elements of the assembled filtering device.